GB 2 078 148 relates to a drop hammer apparatus, wherein a hammer (E) is interconnected with a piston (B) by means of a piston rod. An upright cylinder (A) is open at its upper end, the piston is slidable within the cylinder and the piston rod is slidable through the lower end of the cylinder. The space within the cylinder below the piston is selectively connected to a source (C) of pressurized liquid e.g. water and exhausted by means of a valve (D).
GB 1 397 137 discloses an apparatus for the driving of piles underwater and comprising a hollow tube connected to the pile, the tube being sequentially evacuated by pump and filled with ambient water by opening a valve at the end of the tube, the incoming water, when it strikes the lower end of the tube or any residual water therein producing a driving pulse. The embodiment shown in FIG. 13 involves repetitively and alternately raising a piston (160) with a winch (125) and dropping the piston. Raising of the piston evacuates an enclosure defined by the pile tip and side walls. Quick release of the piston and rapid descent thereof through the pile accelerate a mass of water above the piston. A similar system is shown in U.S. Pat. No. 3,820,346.
GB 2 069 902 relates to a submersible hammer (21) for driving piles comprising a piston (36) and cylinder (35) assembly provided in conjunction with a ram (30) to move the same upwardly when the piston is lifted. Sea water is supplied as power medium at a pressure in excess of the ambient pressure and an inlet valve (50) effects fluid communication between the pressurized sea water and the piston to lift the piston, and thus the ram, and to terminate such communication when the piston reaches a predetermined level. An exhaust valve (51) vents the sea water allowing the piston and ram to fall until the ram impacts the upper end of a pile to drive the same into the sea bed.
GB 1 452 777 relates to a gas discharge powered pile driver comprising an “airgun”. WO 2004/051004 discloses a “pile-driving apparatus comprising a pile, a shoe tip coupled to a toe of the pile, and a drill string disposed within the pile.” U.S. Pat. No. 4,964,473 relates to a method for driving a hydraulic submerged tool, wherein the hydraulic pressure energy is generated in a submerged power converter. U.S. Pat. No. 4,089,165 relates to a water pressure-powered pile driving hammer. The piston of the pile driving hammer is raised by hydraulic (water) pressure. In the underwater pile driving apparatus according to U.S. Pat. No. 4,367,800 the hammer is movable upwards and downwards in a housing which, in operation, is filled with a liquid which is present both above and below the hammer, the hammer being driven at least on the upwards direction by a driving liquid which is pressurized by a motor driven pump located on or adjacent the housing and which is the same as the liquid in which the hammer moves. Other prior art relating to underwater pile driving includes EP 301 114, EP 301 116 and U.S. Pat. No. 4,043,405.
Systems (denoted by numeral 1 in FIGS. 1 and 2) of this type are generally known and usually comprise an impact weight (2), a hydraulic cylinder (3), a piston (4) reciprocatingly accommodated in the hydraulic cylinder (3) and connected to the impact weight (2), high and low pressure accumulators (5, 6), often also referred to as feed and return accumulators (5, 6), a valve system (7) for alternately connecting the hydraulic cylinder (3) to the high and low pressure accumulators (5, 6), a tank (8) for a hydraulic medium, such as hydraulic oil, and a pump (9) for pressurizing the hydraulic medium, i.e. for providing the hydraulic energy required to operate the system.
If the impact weight is accelerated by means of a gas (FIG. 1), a gas spring also known as “cap” (10) is positioned above the piston (4). If the impact weight is accelerated by means of the hydraulic medium (FIG. 2), the valve system (7) comprises a reversing valve for alternately supplying the hydraulic medium to the cylinder spaces above and below the piston (4).
The pressure in and hence the ‘stiffness’ of the system, in particular the pressure in the accumulators and, if present, the gas spring, increases with increasing depth. At extreme depths, such as 1500 meters and deeper, the pressure in the system causes several problems. E.g., it is no longer possible to fill the accumulators from pre-filled gas cylinders. High pressure compressors are required instead.
Further, during acceleration of the impact weight, the pressure in the return accumulator increases to a much greater extent, in turn requiring a higher pressure in the gas spring, if present, and in the feed accumulator. In hydraulically driven systems (FIG. 2), as disclosed in for instance U.S. Pat. No. 4,367,800, to ensure sufficient acceleration at the end of the stroke a very high initial pressure in the feed accumulator is required.
In general, at higher pressures, variations in the operating pressure are amplified, which complicates setting and maintaining the striking energy at a preselected level.
It is an object of the present invention to improve the system according to the opening paragraph.